Light emitting diodes and a method of packaging the same

ABSTRACT

Disclosed herein is a method of assembling an array of light emitting diode (LED) dies on a substrate comprising: positioning dies in fluid; exposing the dies to a magnetic force to attract the dies onto magnets that are arranged at pre-determined locations either on or near the substrate; and forming permanent connections between the dies and the substrate thereby constituting an array of LED dies on a substrate.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the U.S. National Phase Application ofPCT/US2013/037288, filed Apr. 19, 2013 and claims the benefit of U.S.Provisional Application No. 61/687,239 entitled SENSORY LIGHTING SYSTEMfiled on Apr. 20, 2012, the contents of which are incorporated herein byreference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

The present invention was supported in part by a grant from the NationalScience Foundation (Grant No. EEC-0812056) and in part by a grant fromNYSTAR (Grant No. C090145). The United States Government may havecertain rights to the invention.

FIELD OF THE INVENTION

The present invention relates to light emitting diodes (LED's) based oninorganic semiconductor materials and a method for packaging LED's.

BACKGROUND OF THE INVENTION

Current LED packaging typically involves a process where semiconductordevices are positioned on a substrate one at a time. Typical processesmay be limited to a speed of about 10K units per hour. There exists aneed to package these devices, as well as associated control and sensingdevices, into integrated lighting systems at much higher speeds, e.g.,up to 10K units per minute. Some current approaches, including dieprinting and mesoscopic self assembly processes, can be limited by theavailability of manufacturable LED chip designs and defect ratesassociated with high speed placement errors.

SUMMARY OF THE INVENTION

Described herein are programmable directed self assembly and energyassisted placement processes suitable for high speed, high accuracy, andlow defect rate light emitting diode (LED) system packaging operations.

According to one aspect of the invention, a method of assembling anarray of LED dies on a substrate is provided. The method includes thesteps of positioning dies in fluid; exposing the dies to a magneticforce to attract the dies onto the front side of the substrate usingmagnets that are arranged at pre-determined locations either directly onthe backside of the substrate or near the backside of the substrate; andforming permanent electrical and/or mechanical connections between thedies and the substrate thereby constituting an array of LED dies on asubstrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a schematic view of an apparatus for assembling LED's ona substrate.

FIG. 2 depicts a cross-sectional view of a LED die mounted to a tapebacking.

FIG. 3 depicts an exemplary process sequence for fabricating a buoyantlayer of a die.

FIG. 4 depicts a schematic view of another apparatus for assemblingLED's on a substrate that is shown schematically.

FIG. 5 is a flow chart that depicts the process steps for assemblingLED's on a substrate.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 depicts an apparatus 1 for assembling LED dies on a substrateaccording to a first exemplary embodiment of the invention. A die is anindividual chip that diced from a device wafer, where the device wafermay be a large piece of a semiconductor. Apparatus 1 includes acontainer 4 for storing fluid, such as water. Feeder 5 is positionedabove container 4 for distributing buoyant LED dies 2 onto surface 3 ofthe fluid. Feeder 5 may be a funnel, vibration feeder or a rotaryfeeder, for example.

Substrate 7, which is flexible and wound around and stored in substratereel 6, is wound about cog 8, magnetic roller 10, cog 12, cog 14 andreel 16. Substrate 7 is optionally composed of polyethyleneterephthalate having a thickness of 0.127 mm/5 mil. Other transparentmaterials for use as substrate 7 with good transparency in the visibleor UV would include flexible glass or polymers, including but notlimited to polyethylene terephthalate, polycarbonate or other likematerials. Substrate 7 may also contain color converting materials usedto covert LED light to other wavelengths. Alternatively, a highlyreflective material comprised of a dielectric reflector and/or metalreflector may be used. Various uses as required by the optical designand performance of the system can be considered.

Although not shown, one or more of reel 6, cog 8, magnetic roller 10,cog 12, cog 14 and reel 16 may be connected to a stepper motor (notshown), which causes rotation of the driven roller/cog/reel about itsaxis of rotation (the axis of rotation extends into the page). It shouldbe understood that, in operation, substrate 7 moves from reel 6 to reel16.

Screen 15, which resides at the edge of container 4 adjacent magneticroller 10, has a series of apertures 17 (one shown). Apertures 17 aresized to position dies 2 in the proper orientation before dies 2 areloaded onto substrate 7.

Magnetic roller 10 is positioned at the edge of container 4 adjacentscreen 15. Magnetic roller 10 includes an array of magnets 19 that arearranged on its periphery. Magnets 19 facilitate the magnetic pickup ofdies 2 on substrate 7. According to one exemplary embodiment, thirtycube-shaped magnets 19 are arranged on roller 10. Each magnet 19 may bea nickel-plated Neodymium N42 magnet that is distributed commercially byK&J Magnetics of Jamison, Pa., USA. Electromagnets may also be used.

According to one exemplary embodiment, each magnet 19 has a side lengthof 1.588 mm ( 1/16 inch) that is embedded into a neoprene sheet 19′ ofthe same thickness, which is wrapped around roller 10. The size andpitch of magnets 19 may vary, and are dictated by the LED die size anddensity of LED array needed, depending on application requirements.Openings in neoprene sheet 19′ can be formed using a CO2 laser with anedge-to-edge distance of 10 mm (pitch≈10.79 mm) in the horizontaldirection (i.e., perpendicular to direction of web movement) and anedge-to-edge distance of 13 mm (pitch≈13.79 mm) in the direction of webmovement. A thin layer of transparent adhesive tape 19″ is applied ontop of magnets 19 to provide a smooth outer surface on the roller 10.

According to one exemplary method of operating apparatus 1, feeder 5distributes buoyant LED dies 2 onto surface 3 of the fluid. Magnet 9 isactivated causing LED dies 2 to temporarily sink toward the bottom ofcontainer 4. Once at or near the bottom of container 4, magnet 9 isdeactivated and dies 2, which are buoyant, assume the proper top/bottomorientation as they rise and resurface onto top surface 3 of the fluid.Vibrator 18 may be employed to vibrate the fluid within container 4 tofurther facilitate proper top/bottom orientation of dies 2 as dies 2resurface.

A slight fluid current in the fluid causes dies 2 to float towardmagnetic roller 10 once they resurface. The current is created by pump11, which draws fluid from the bottom end of container 4 beneathmagnetic roller 10, and delivers the fluid to the top end of container 4at a location opposite magnetic roller 10. Pump 11 is an optionalcomponent of apparatus 1, as the water current provided by pump 11 maybe disadvantageous.

Substrate 7 travels along magnetic roller 10 and passes by the edge ofcontainer 4. Substrate 7 may be at least partially positioned withincontainer 4 in contact with fluid surface 3. The magnetic regions onroller 10 attract dies 2. The magnetic force causes dies 2 to passthrough apertures 17 of screen 15 and attach onto substrate 7. Aperture17 ensure that dies 2 are oriented properly on substrate 7. At thispoint in the process, dies 2 are releasably and magnetically attached tosubstrate 7.

Substrate 7 with dies 2 releasably attached thereto are directed overcog 12 and are exposed to thermal energy in the form of heat, which canbe generated by, for example, infrared (IR) lamp 13. The heat generatedby lamp 13 causes solder on the bottom surface of dies 2 to melt andadhere to substrate 7, thereby forming an electrical and mechanicalconnection between dies 2 and substrate 7. The solder may be provided inthe form of bumps on the bottom surface of dies 2. Once the soldercools, dies 2 are permanently attached to substrate 7. Item 13 mayalternatively represent a reflow oven.

Substrate 7 with dies 2 permanently attached thereto are directed overcog 14 and are wound about reel 16. Substrate 7 with dies 2 permanentlyattached thereto form a sheet of LED's. Once reel 6 is emptied and reel16 is filled, reel 6 is replaced with a full reel, and reel 16 isremoved from apparatus 1 and is ready for use as a sheet of LED's.

Apparatus 1 may vary from that which is shown and described. Forexample, magnets may be positioned on the front side or backside ofsubstrate 7 instead of roller 10. Also, several components of apparatus1 are optional, such as vibrator 18, pump 11 and magnet 9.

Although not shown, substrate 7 may contain optical and electricalstructures (e.g., phosphors, lens structures, and electricalinterconnects) that are formed prior to assembling dies 2 onto substrate7. Dies 2 could be placed on substrate 7 so that they register withthose preexisting features of substrate 7. Substrate 7 could also beused to create laminated die structures that could be subsequentlyprocessed (e.g., laser drilling, inkjet electrical trace printing,phosphor deposition, etc.). Regardless of the process, thin “sheets oflight” could form the high volume, low cost foundation of a wide rangeof novel illumination systems that would mimic organic light-emittingdiode (OLED) technology, but with distinguished advantages over OLED.Further details regarding OLED technology may be found athttp://en.wikipedia.org/wiki/OLED, for example.

FIG. 2 depicts a cross-sectional view of die 2 of FIG. 1. Die 2 is shownreleasably mounted to tape release layer 24 and encapsulated inencapsulant 25. Referring now to the features of die 2, each die 2 maybe fabricated from a Silicon wafer 21 having a thickness of 200 μm, forexample. A Silicon die may be used for developing the process. ForGallium nitride (GaN) based LED packaging, wafer 21 may be made of GaNor may include GaN on its substrate, such as sapphire or 4H SiliconCarbide (SiC), or silicon. The thickness of die 2 may also vary,depending on whether or not a substrate is employed.

One side of wafer 21 includes a magnetic layer 22. Magnetic layer 22 maybe composed of a 1.65 μm thick nickel film deposited using electron beamevaporation from 99.999% pure Ni pellets. Magnetic layer 22 can bedeposited using various techniques, such as electroplating or e-beamevaporation. Screen printing can also be used for creating embeddedpowder magnets. Nickel is compatible with common LED fabricationprocesses and is a promising material for use as a magnetic layer.

The opposite side of wafer 21 is coated with a thick, lightweight andbuoyant layer 23 that is designed to provide buoyancy to die 2 and helporient it in correct top/bottom orientation on fluid surface 3. Buoyantlayer 23 should have a low density, be mechanically robust to survivedicing and handling of die 2, and have low water absorption whensubmerged in water. By way of example, buoyant layer 23 may be composedof syntactic foam. Porous materials may be undesirable if they aresusceptible to water absorption.

According to one exemplary method for creating buoyant layer 23, hollowglass microspheres are first mixed with organic matrix material. Withregard to the hollow glass microspheres, one example of a suitablematerial is 3M™ Glass Bubbles K1, which is made commercially availableby 3M™ Energy and Advanced Materials. The 3M™ Glass Bubbles K1 areparticularly suitable due to their low density (0.125 g/cc) and highisostatic crush strength (250 psi). Properties of this material arelisted in Table 1.

TABLE 1 Properties of glass microspheres from 3M ™ Glass Bubbles. NameK1 Glass Bubbles Composition Soda-lime-borosilicate glass Target crushstrength 250 (90% survival, psi) True Density (g/cc) 0.125 Distributionof 10th % 30 particle size 50th % 65 (microns, by volume) 90th % 115Color (unaided eye) White

With regard to the organic matrix material, one example of a suitablematerial is Benzocyclobutene (BCB), which is made commercially availableby the Dow Chemical Company™ under the trade name CYCLOTENE™ 3022-57.S1813 photoresist may also be used.

FIG. 3 depicts an exemplary process sequence for fabricating buoyantlayer 23 of die 2. According to one exemplary method of fabricatingbuoyant layer 23, at step 30, an adhesion promoter is applied to wafer21 of die 2. At step 32, an organic matrix is spun coat onto the waferat 1000 rpm for 30 seconds. At step 34, glass microsphere's are spuncoat onto the wafer at 1000 rpm for 30 seconds. The glass microspheresand the organic matrix are deposited using spin-coating in separatesteps because pre-mixing them may not provide a homogenous mixture dueto the buoyancy of the microspheres. At step 36, the wafer is baked at155 Celsius for 2 minutes. Thereafter, steps 32, 34 and 36 are repeatedin sequence four times. At step 38, the wafer is baked at 155 Celsiusfor 2 minutes. The following parameters can be optimized to create alow-density buoyant layer that is mechanically robust: matrix material,spin-coating parameters, number of spin-coating steps, and baketemperature and duration. One or more steps of this process can berepeated to achieve the desired thickness of buoyant layer 23.

After the buoyant layer deposition phase, the samples are mounted ondicing tape 24 with magnetic layer 22 of die 2 arranged face down. Tape24 is diced into square dies, each having a side length of 1.6 mm, whichis a size close to typical size of power LEDs used in lightingapplications.

Considering the large amount of LED dies per wafer, it is desirable toavoid or minimize individual handling of dies 2 by releasing dies 2 fromthe dicing tape 24 using acetone, for example, or another solvent. Toprevent the solvent from degrading buoyant layer 23, dies 2 are coveredin an encapsulant 25. Individual dies 2 can either be coated by a thinlayer of encapsulant 25 without being connected to each other, or theycan form a “pseudowafer” containing all dies 2 on a tape connected by alarge amount of encapsulant 25.

After the application of encapsulant 25, dicing tape 24 with dies 2 issubmerged in a solvent bath (or sprayed with solvent). The solvent bathis separate from the water bath in container 4. After being exposed tosolvent for several minutes, dies 2 are removed from the solvent bath.Thereafter, dies 2, which are still embedded in encapsulant 25, areremoved from dicing tape 24 (by peeling or washing, for example) andtransferred to a deionized water bath to dissolve encapsulant 25. Thedeionized water bath is separate from the water bath in container 4. Thetemperature of the deinonized water bath may be elevated to 80-90degrees Celsius, for example, and/or include ultrasound to acceleratethe dissolution of encapsulant 25. When the buoyant force acting on dies2 overcomes the adhesion to encapsulant 25, dies 2 detach fromencapsulant 25 and rise to the surface of the deionized water bathleaving the remaining encapsulant 25 on the bottom of the deionizedwater bath. Encapsulant 25 may be dissolved completely.

After the aforementioned process is completed, dies 2 can be transferredinto feeder 5 of FIG. 1. Dies 2 may be cleaned with Isopropyl alcohol,or any other solvent that does not affect buoyant layer 23, and driedbefore they are transferred to feeder 5.

It should be understood that encapsulant 25, the solvent bath, and thedeionized water bath are optional features of the invention.

FIG. 4 depicts an apparatus 50 for assembling LED's on a substrateaccording to a second exemplary embodiment of the invention. Theembodiments of FIGS. 1 and 4 are similar, and the primary differencesbetween those embodiments will be described hereinafter. Apparatus 50includes a container 39 for storing fluid, such as water.

Plate 43 having an array of magnets 44 is positioned in container 39 andimmersed in the fluid. Mask 40 having apertures 41 is positioned aboveplate 43, such that magnets 44 register with apertures 41. Substrate 42,which is analogous to substrate 7 of FIG. 1, is sandwiched between plate43 and mask 40. It should be understood that plate 43, mask 40 andsubstrate 42 are shown partially exploded in FIG. 4, and only a portionof dies 2 are shown.

In operation, magnets 44 are activated to cause dies 2 to sink towardthe bottom of container 39. Due to the force of magnets 44, dies 2 landand orient themselves within respective apertures 41. Plate 43, mask 40and substrate 42 are then removed from container 39. Thereafter, plate43 and mask 40 are separated from substrate 42. Dies 2 remain in theirrespective positions on substrate 42.

Thereafter, substrate 42 and dies 2 are exposed to thermal energy in theform of heat generated by an infrared (IR) lamp 45. The heat generatedby lamp 45 causes solder on the bottom surface of dies 2 to melt andadhere to substrate 42, thereby forming an electrical and mechanicalconnection between dies 2 and substrate 42 to form a sheet of LED's.

FIG. 5 is a flow chart that depicts an exemplary method for assemblingLED's on a substrate. It should be understood that the method depictedin FIG. 5 is not limited to any particular step, or sequence of steps.Also, the method depicted in FIG. 5 applies to apparatuses 1 and 50 ofFIGS. 1 and 4, respectively.

According to the exemplary method, at step 60, magnetic layer 22 isformed on the respective dies 2. Magnetic layer 22, which may becomprised of nickel electrodes, can be formed using electron beamevaporation.

At step 62, buoyant layer 23 is formed on the respective dies 2. Buoyantlayer 23 may be composed of syntactic foam. Deposition of buoyant layer23 can be accomplished using either spin-coating in several steps usingthe process sequence shown in FIG. 3, or using alternative methods suchas molding or stencil printing. While glass is the most common materialfor the hollow microspheres of buoyant layer 23, use of other materials(e.g. phenolic hollow microspheres) is also possible.

At step 64, dies 2 are taped, diced and binned, as previously describedwith respect to FIG. 3.

At step 66, dies 2 are released from the dicing tape using solvent. Ifdies 2 include encapsulant 25, then dies 2 are immersed in a deinonizedwater bath to dissolve encapsulant 25.

At step 68, dies 2 are transferred into fluid within container 2 usingfeeder 5, for example.

At step 70, dies are assembled onto substrate 7 using a magnetic force,using the apparatuses 1 or 50 of FIGS. 1 and 4, respectively. This stepof the process is referred to herein as directed self-assembly.

At step 72, permanent electrical connections are formed between dies 2and substrate 7. Solder bonds can be used if electrical traces onsubstrate 7 were formed prior to the assembly of dies 2. Inkjet printingof conductive traces (e.g. silver nanoparticle ink) and laser drillingcan be used if the traces are formed after the assembly of dies 2 on thereceiving substrate 7. Alternatively, a die assembled according to step70 may be laminated with a second optical material before electricalcontact formation, with subsequent electrical contact formation madethrough laser drilling a via for printed or plated electricalconnections to the buried die.

At step 74, buoyant layer 23 is dissolved. This step can be accomplishedusing organic solvents which are capable of dissolving either the matrixof buoyant layer 23 or the (optional) adhesive layer that keeps buoyantlayer 23 attached to the die surface.

At step 76, phosphors are deposited on dies 2, and then dies 2 areencapsulated. This step can be accomplished either after buoyant layer23 is dissolved, or before the buoyant layer 23 is deposited on die 2 atstep 62. Thereafter, dies 2 on substrate 7 constitute a sheet of LED'sthat are ready for use.

Test Results: Introducing Dies into the Fluid

Introducing dies to the fluid with correct top/bottom orientation is aprerequisite for the pickup of dies on a flexible substrate. Severalmethods were tested to determine the best way to achieve the correcttop/bottom orientation of the dies. The goal of the test was to achievea high percentage (referred to as yield) of dies floating with correcttop/down orientation, i.e., buoyant layer oriented on top, the magneticlayer oriented on the bottom and the angle between the die and waterssurface less than 90 degrees. Yields for the following methods werecompared during the test:

-   -   A. Dropping dies 2 from 30 cm onto the fluid surface 3 through a        funnel to allow dies to orient themselves during the fall;    -   B. Sinking dies 2 with a magnet 9 located at the bottom of a        shallow container and removing or deactivating magnet 9; and    -   C. Sinking or wetting dies 2 in one container with fluid and        transferring the dies with a non-magnetic mesh (i.e., aluminum        mesh with a wire diameter of 0.4572 mm and openings of 1.12 mm)        to another container.

The surface tension of the fluid within container 4 can play animportant role in the process because surface tension of the fluidaffects the way dies 2 are wetted in the fluid and can make the diesfloat, which is undesirable. Three options were explored during thetest:

-   -   A. Introduction of dies 2 into deionized (DI) water;    -   B. Introduction of dies 2 into isopropyl alcohol (IPA) followed        by transfer of the IPA-wetted dies into DI water; and    -   C. Introduction of dies 2 into water containing a surfactant of        Sodium Dodecyl Sulfate (SDS) with a molar concentration of 0.01        M, which is slightly above critical micelle concentration of        0.008 M.

Yields were compared for the methods of introducing dies into the fluid,as shown in Table 2. In each case 500 data points (dies) were used.Tests were conducted in 22×18 cm container with 1 liter of fluid. Thedie surface was cleaned with IPA and dried before each test.

TABLE 2 Yields for different methods introducing dies into the carrierfluid (500 dies were used in each test). Method for introducing diesinto Yield Dropping from 30 cm into DI water 95.4% Dropping from 30 cmwater with 0.01M of SDS 97.8% Sinking with a magnet in a shallowcontainer 99.2% with DI water and releasing the magnet Dropping from 30cm into IPA and 96.2% transferring with a mesh to another container withDI water Sinking with a magnet in a shallow container 99.8% with watercontaining 0.01M of SDS and releasing the magnet Sinking with a magnetin a shallow container  100% with water containing 0.01M of SDS andtransferring with a mesh to another container with 0.01M of SDS in water

Three important observations resulting from the test are summarizedhereinafter. First, within several seconds after the IPA wetted dieswere transferred into the water, the dies 2 moved in fast abruptthrusts, often accompanied by rotation of the dies. This behavior canalso be observed if the dies are wetted with methanol or acetone. Thisbehavior may be attributed to the local decrease of surface tension dueto the presence of IPA.

Second, few dies 2 were trapped below other dies. Agitation or vibrationallowed any trapped dies to escape and reach the water surface. Therewas no noticeable impact of this die behavior on the assembly process.

Third, sinking dies 2 using a magnet 9 offers the best results, andtransferring the dies using a mesh allows using this method even whenlimitations of the setup do not allow placing a magnet directly belowthe container, where the next step (i.e., pickup on a flexiblesubstrate) is performed.

Test Results: Magnetic Force-Assisted Die Assembly

Once dies 2 are transferred to the fluid in container 4, they areassembled on flexible substrate 7, as described previously. Effects ofthe key process parameters are evaluated and summarized as follows:

-   -   A. Angle between substrate and the container: an angle of close        to 90 degrees between the substrate and the container can be        beneficial in reducing the accidental pickup of dies on the        substrate.    -   B. Substrate movement speed: a lower speed in the range of 1-10        mm/s results in pickup of multiple dies with one magnet.        Increasing substrate speed to the range of 50-100 mm/s can        resolve this issue, but high substrate speed makes the reliable        die supply to the interface a more difficult task.    -   C. Vibration of water surface: a low-frequency vibration (7-10        Hz) of the water surface provides a rejection mechanism during        the die pickup and reduces the amount dies that are picked up in        the wrong positions. High frequency vibrations (e.g., 1 G        vibration at 12000 rpm by a vibrating disk motor) do not have        any measurable impact on the process.    -   D. Magnet material: the strength of a Neodymium (Nd) magnet is        adequate for the pickup process. If a lower surface tension is        used instead of vibrations as a die rejection mechanism, then        the attraction by these magnets is too strong and can be reduced        by partial demagnetization at elevated temperatures above 140        degrees Celsius.    -   E. Water flow: assembly at zero water flow results in more        reproducible results. The zero water flow reduces uncertainty        caused by turbulent flow of the fluid in the container, and        helps to control surface tension more reliably. The fluid in the        container between test runs can be refilled with a syringe. Even        at zero water flow, the substrate draws certain amount of fluid        as it moves, providing additional attraction force for the dies        to the substrate.    -   F. The die pickup process can be improved by placing a screen        (such as screen 15) with vertical slits or apertures 17 in front        of the substrate to reduce the possibility of picking up more        than one die by each magnet. An aperture width of 2.4 to 3 mm        allows dies to pass without obstruction, while preventing pickup        of dies to the left and right of the magnets of the roller head.    -   G. The surface tension of the fluid in the container manifests        itself through the following effects: it increases the chance of        die pickup on the surface, reduces the chance of air bubble        formation on the fluid surface, and helps to attract the dies to        the interface and create reliable supply of dies for magnetic        pickup on a substrate. A lower surface tension is preferable as        the benefits of lowering the force, which competes with magnetic        pick up, outweigh the advantages provided by the surface        tension. Measured surface tension of 42 dyn/cm (for water with        0.01M of SDS) is low enough to prevent accidental pickup and is        used an alternative to vibration as a die rejection mechanism.

Table 3 summarizes the assembly parameters that were optimized forassembly yield. The achievable yield is 86%, which is defined as theratio between the amount of dies picked up at the correct locations tothe total amount of dies picked up.

TABLE 3 Parameters for magnetic die pickup process. Parameter ValueAngle between the 90° substrate and cartridge Substrate material Kapton500 HPPST Magnet material NdFeB N42, partially demagnetized at 180degrees Celsius Substrate speed 50 mm/s Surface tension of 42 dyn/cmcarrier fluid Vibration frequency 10 Hz Slit width 2.8 mm

Test Results: Summary and Conclusions

The testing shows that the directed self-assembly technique enableshigh-speed assembly of small semiconductor dies on a large area flexiblesubstrate in a roll-to-roll process.

Fluidic handling of dies can be used as an alternative to serialmechanical handling. The fluidic handling methods described herein arebased on floatation of dies on the fluid surface. Die floatation isachieved by creating a layer consisting of hollow glass microsphere inseveral spin-coating steps. Assembly of dies having correct top/downorientation on the fluid surface can be accomplished with high yield.Magnetic die pickup is a fast way for assembling dies on large areasubstrates. The wide parameter space allows numerous options for processoptimization to improve the die placement yield, particularly forassembly of specially designed LED's.

Directed Self Assembly (DSA) using magnetic force and a fluid medium isa perspective replacement for pick-and-place LED assembly. The use ofmagnetism, instead of electrostatics, reduces the risk of electrostaticdischarge (ESD) damage to the components during assembly. Magnetism isalso less sensitive to the surface properties of the component and thesubstrate.

While preferred embodiments of the invention have been shown anddescribed herein, it will be understood that such embodiments areprovided by way of example only. Numerous variations, changes andsubstitutions will occur to those skilled in the art without departingfrom the spirit of the invention. Accordingly, it is intended that theappended claims cover all such variations as fall within the spirit andscope of the invention.

What is claimed:
 1. A method of assembling an array of light emittingdiode (LED) dies on a substrate comprising: positioning dies in fluid;exposing the dies to a magnetic force to attract the dies onto a frontside of a substrate using magnets that are arranged at pre-determinedlocations either directly on the backside of the substrate or near thebackside of the substrate; and forming permanent electrical and/ormechanical connections between the dies and the substrate therebyconstituting an array of LED dies on a substrate, wherein prior to thepositioning, the method comprises forming a buoyant layer on one side ofeach of the dies.
 2. The method of claim 1, wherein prior to thepositioning, the method comprises forming a magnetic layer on anotherside of each of the dies.
 3. The method of claim 1, wherein after thepositioning, the method further comprises temporarily sinking the diesusing another magnetic force and then deactivating the magnetic force sothat the dies rise to a top surface of the fluid with the buoyant layerfacing in an upward direction.
 4. The method of claim 1, wherein themethod further comprises dissolving the buoyant layer.
 5. The method ofclaim 1 further comprising advancing the substrate with respect to thefluid to continuously expose the dies to magnetic forces andcontinuously attract the dies onto respective ones of the magnets thatare arranged at pre-determined locations either directly on the backsideof the substrate or near the backside of the substrate.
 6. The method ofclaim 1, wherein the forming comprises exposing the dies and thesubstrate to an elevated temperature to cause solder on a surface of thedies to adhere to the substrate.